id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
13518,"Default constructor is explicit, no reason to be.",anonymous,Joaquín M López Muñoz,"See the follwing code
{{{#!c++
#include <boost/multi_index_container.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <boost/multi_index/identity.hpp>

using namespace std;

struct aggregate {
	boost::multi_index_container<int> a;
};

int main()
{
	using boost::multi_index_container;
	multi_index_container<int> a; // default init
	// multi_index_container<int> b(); // NOT A THING, not object init
	multi_index_container<int> c =
		multi_index_container<int>(); //value init + (elided) copy

	auto d = multi_index_container<int>(); // value init + (elided) copy

	multi_index_container<int> e{}; // direct-list-init, chooses value init

	auto f = multi_index_container<int>{};
	// direct-list-init, chooses value init
	// followed by elided copy

	multi_index_container<int> g = {}; // ERROR
	// copy-list-init, should choose value init
	// but fails becase default constructor is explicit

	aggregate agg1{};    // ERROR for the same reason above
	aggregate agg2 = {}; // ERROR for the same reason above
	// Does aggregate init which does copy-list-init on members that
	// were not specified in the list.
	
	auto agg3 = aggregate(); //OK, value init

	multi_index_container<int> h{1, 2, 3, 4};
	// direct-list-init, chooses initializer list

	multi_index_container<int> i = {1, 2, 3, 4};
	// copy-list-init, chooses initializer list
	return 0;
}
}}}

The problematic ctor is in line 175 of multi_index_container.hpp
{{{#!c++
  explicit multi_index_container(

#if BOOST_WORKAROUND(__IBMCPP__,<=600)
    /* VisualAge seems to have an ETI issue with the default values
     * for arguments args_list and al.
     */

    const ctor_args_list& args_list=
      typename mpl::identity<multi_index_container>::type::
        ctor_args_list(),
    const allocator_type& al=
      typename mpl::identity<multi_index_container>::type::
        allocator_type()):
#else
    const ctor_args_list& args_list=ctor_args_list(),
    const allocator_type& al=allocator_type()):
#endif

    bfm_allocator(al),
    super(args_list,bfm_allocator::member),
    node_count(0)
  {
    BOOST_MULTI_INDEX_CHECK_INVARIANT;
}
}}}
Probably, it was made explicit for the case when it is called with non-default parameters.",Bugs,closed,To Be Determined,multi_index,Boost 1.66.0,Problem,fixed,,